Strange Signal at Galactic Center-Is It Dark Matter?

Artist's conception of the Milky Way surrounded by a dark matter halo. Image: ESO/L. Calçada

Are there dark doings near the center of the Milky Way? That may be so when it comes to the collision of dark matter particles. Although such particles are invisible, we could still theoretically see the mess they make when they collide. It's this idea that leads physicists to scour the galaxy for some glimmer of dark matter collisions. Spot a line produced by a pair of gamma-rays emanating from just the right spot and you may have found coveted clues to the dark matter mystery.

Now a collaboration of scientists using the Fermi Gamma-Ray Spacecraft's Large Area Telescope instrument (Fermi–LAT) has confirmed seeing an unusual gamma-ray line near the galactic center. If the finding stands up to further scrutiny, it's possible this line comes from the annihilation of dark matter.

In April theoretical physicist Christoph Weniger, now at the GRAPPA Institute in Amsterdam, analyzed Fermi–LAT's publicly available data and spotted a strange gamma-ray line near the galactic center. There's no known astrophysical event that can tidily explain this line—but the collision of dark matter particles might. If that were the case, it would be a major discovery: Once physicists spot the products of such an annihilation, they could begin to understand the particles that collided.

But there was a catch: Weniger is not a member of the Fermi-LAT collaboration and therefore cannot be able to account for the quirks of their instrument. What was needed was a weigh-in from Fermi-LAT collaboration physicists; they know the data best and would be able to confirm any hint of dark matter.

That confirmation—though very qualified—came earlier this month at the Fourth International Fermi Symposium. "We do see a line at the galactic center but at lower significance than others have seen," The Ohio State University physicist and collaborator Andrea Albert says.

Specifically, when Albert and colleagues looked within a certain region of the galactic center, they saw a prominent gamma-ray line with an energy of 135 giga–electron volts (GeV) with 3.35 sigma local significance.* This is close enough to Weniger's observations to infer that this is the line he spotted as well. There’s even a second, much less pronounced line that could support the conclusion that this is dark matter. If dark matter really is what’s producing the gamma-ray line in the galactic center, physicists would predict that it could produce additional signals nearby, annihilating and releasing other particles as well as gamma-rays. The Fermi-LAT collaboration has some evidence of a second line that meshes well with this theoretical prediction.

But there is a problem with the 135 GeV gamma-ray line. When Fermi–LAT physicists looked at data from the Earth limb, or outer rim of the atmosphere, they found another linelike bump in the data at 135 GeV. Put simply, that region should be bump-free. The presence of a line at the same energy level in two different data sets suggests that these could be aberrations introduced by data processing.

Then again, the Earth-limb finding could just be a coincidence—albeit an eerie one."For now, we cannot exclude the possibility that the line in the galactic center is a dark matter line," Albert says. "Within a year we hope to have an answer."

*Clarification (11/19/2012): This sentence was edited after posting. The line's 3.35 sigma only reflects local significance; its global significance was less than 2 sigma.

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